Norcal is a well-established company, with a decent reputation for a company doing primarily IFTs. About the same as most other companies from what I hear, but someone else may be able to provide more info.
On the other hand, United or United Plus is a horrible place to work. They have tons of craigslist ads up, and will interview just about anyone. Its not hard at all to get a job, there but there's one big catch. You won't be working on a BLS ambulance. At this time, United only has 1-2 BLS rigs and runs only a few BLS calls at a time. The remaining 12 or so rigs are wheelchair or gurney vans. Their reason for wanting "EMTs" is that their slogans/advertisements read, "All attendants and drivers are certified EMTs".
United's pay is ~$10/hr, no comission, and hours/ shifts do get cut quite often. From what I gather, the people that get on the BLS rigs are usually the ones who suck up/ are favorites of the owner "Chris", and are given the "privelage" of working on their their single BLS rig. During the interview, you are usually told that you will be working on a BLS rig after a month of gurney/wheelchair transports, but that is hard to believe.
Please let me know if you'd like more info.